Copper ternary chalcogenide compounds and alloys are among the most promising light-absorber materials for solar cell applications and polycrystalline copper indium selenium (often with indium partly replaced by gallium) (CIGS) thin-films, due to their direct (and tunable) energy band gaps, very high optical absorption coefficients in the visible to near-infrared (IR) spectrum range and high tolerance to defects and impurities (see, for example, H. W. Schock et al., “CIGS-based Solar Cells for the Next Millennium,” Prog. Photovolt. Res. Appl. 8, 151-160 (2000)), have been the focus of extensive investigation for over two decades. The highest efficiency CIGS-based solar cells are traditionally produced by vacuum-based techniques, which require sophisticated vacuum deposition equipment, high processing temperatures (typically above 450 degrees Celsius (° C.)), and usually a post-deposition treatment in a selenium (Se)-rich atmosphere (such as Se vapor or hydrogen selenide/sulfur (H2Se/S)).
However, in an effort to reduce fabrication costs and extend the range of substrates upon which CIGS devices can be integrated, there have been significant efforts to develop lower-temperature, ultra-low-cost solution-based approaches for the fabrication of the CIGS absorber layer for solar cells. A typical challenge faced by these solution-based deposition methods is the difficulty in controlling the grain structure of the CIGS absorber layer. Poor grain structure severely limits solar cell efficiency. Namely, grain boundaries can act as recombination centers for the photogenerated electrons and holes, which are detrimental to the device performance. In general, grain sizes on the order of absorber layer thickness (micrometer (μm)-length scale) are desirable in order to minimize such recombination effects.
Thus, to date the best cell efficiencies achieved from solution-based CIGS absorber growth techniques have reached from about seven percent (%) to about nine % for electrochemical deposition (seven % for simultaneous deposition of all elements and nine % for deposition of metals followed by selenization) (A. Kampmann et al., “Electrodeposition of CIGS on Metal Substrates,” Mat. Res. Soc. Symp. Proc. Vol. 763, B8.5.1-B8.5.6 (2003); lower than five % for spray pyrolysis/spray chemical vapor deposition (CVD) (S. Duchemin et al., “Studies on the Improvement of Sprayed CdS—CuInSe2 Solar Cells,” Proceedings of the 9th EPVSEC, Freiburg, Germany, 476-479 (1989); and J. A. Hollingsworth et al., “Single Source Precursors for Fabrication of I-III-VI2 Thin-Film Solar Cells Via Spray CVD,” Thin Solid Films vol. 431-432, 63-67 (2003); and as high as 13.7% in very small area devices (0.08 square centimeter (cm2)) using nanoparticle-precursor deposition method (V. K. Kapur et al., “Fabrication of CIGS Solar Cells Via Printing of Nanoparticle Precursor Inks,” Proceedings of the DOE Solar Program Review Meeting, DOE/GO-102005-2067, 135-136 (2004)).
Attempts have been made to enhance solar cell performance in closely-related chemical systems (either gallium (Ga)-free CuInSe2 (CIS) or sulfide instead of selenide, i.e., CuInS2). See, for example, U.S. Patent Application No. 2005/0056863 filed by Negami et al., entitled “Semiconductor Film, Method for Manufacturing the Semiconductor Film, Solar Cell Using the Semiconductor Film and Method for Manufacturing the Solar Cell” (controlling carrier density in a, e.g., CIS light-absorption layer of a solar cell); U.S. Patent Application No. 2005/0194036 filed by Basol, entitled “Low Cost and High Throughput Deposition Methods and Apparatus for High Density Semiconductor Film Growth” (adding, at impurity levels, liquid materials, such as sodium (Na), potassium (K), lithium (Li), phosphorous (P), antimony (Sb) and bismuth (Bi), to an absorber layer to improve the quality of the absorber layer); International Patent Application No. WO2008/013383 filed by LG Chem, LTD., entitled “Method for Preparing CIS Compounds and Thin Layer, and Solar Cell Having CIS Compound Thin Layer” (dopants, such as Na, K, nickel (Ni), P, arsenic (As), Sb and Bi can be used to enhance a CIS absorption layer of a solar cell) and Y. Akaki et al., “Structural and Optical Characterization of Sb-Doped CuInS2 Thin Films Grown by Vacuum Evaporation Method,” Journal of Physics and Chemistry of Solids, 64, 1863-1867 (2003) (describes structural, electrical and optical properties of Sb-doped CuInS2 thin films without device results). None of these attempts, however, provide an implementable solution to the problem of charge recombination at grain boundaries. Namely, none of the above references directly address with any specifics the grain structure of a CIGS absorber layer and how the grain structure relates to the performance of a solar cell.
Therefore, techniques to improve the grain structure and thereby the performance of CIGS-based solar cells would be desirable.